Recent improvements in the performance of color copiers have been accompanied by the development of functions which prevent the copying of banknotes, securities such as stock certificates and bonds, commuter passes, prize tickets and gold notes. (These articles shall be referred to as “specific documents” below.)
One method known as a technique to prevent the copying of a specific document includes comparing read image data with image data representing the specific document stored in memory in advance, determining based upon the comparison whether the image is one that may be copied, and executing processing on the printer side in such a manner that the image will not be printed out normally if the result of the determination is that copying is not allowed.
Recently, however, advances in networking and electronic filing have been made and images in the form of digital information are now being exchanged over networks. As a consequence, it is difficult to prevent unlawful copying of digitized copyrighted documents and images merely by a copying preventing function provided on the printer side.